zed to date below. A placental cDNA library was constructed in the expression vector lambda gt?11?. Poly A?+? RNA was isolated from a 34 week-old placenta and used for the synthesis of double-stranded cDNA. The cDNA was inserted at a unique Eco RI site in the structural lac Z gene of the bacteriophage lambda, giving rise to insertional inactivation of endogenous viral beta galactosidase activity and synthesis of lac z-insert-coded fusion polypeptide in the recombinant phages. A library containing 10?6? independent recombinants was generated containing inserts with a minimum size of 1.4 kb. Rabbit polyclonal antibodies were generated against CNBr fragments of purified PLAP in an attempt to increase the proportion of antibody molecules reactive with continuous epitopes. These antibodies reacted well in Western blots. A screening of approximately 1.2 x 10?5? recombinant phages resulted in the identification of 12 antibody-positive clones. The sizes of the cloned PLAP inserts ranged from 1.4 kb to 3.0 kb, and cross-hybridizations indicated that they were all related. Hybridization with a mixed oligonucleotide probe, derived from the amino-terminus of the PLAP molecule, recognized only the 3.0 kb insert. In turn, the nick-translated 3.0 kb insert hybridized in Northern blots with an RNA species of approximately 3.2 kb, indicating that the cloned insert represents full or nearly full length PLAP cDNA. (M)
